For many years, there has been a strong desire to develop gas chromatographic detectors that detect only specific elements. It is well known that a gas chromatographic column is able to separate very similar compounds into separate peaks as a function of time. This time-based separation is especially useful in delineation of adjacent peaks. Once the peaks are separated, it is necessary to identify the constituents of the peaks. Then the concentrations of the compounds within the sample gas input into the GC can be determined.
Among the different types of detectors used for a gas chromatograph, electron capture detectors (ECD) are useful for the detection of electron attaching compounds, such as halogens and nitro compounds. ECD has been used as a GC detector for more than four decades, because it offers the highest sensitivity to electron-capturing compounds. This selective sensitivity to halides makes the detection method especially valuable for the trace analysis of many environmentally important organic compounds such as pesticides. ECD is the only detector that detects CFCs and chlorine-containing pesticides at trace levels.
Conventional ECDs usually use a radioactive ionization source in form of 63Ni foil, e.g. in U.S. Pat. No. 4,063,156. Using such ionization source has some benefits, mostly because of simplicity, stability, noise-free and no need to extra power for ionization. However, there are some real problems associated with using radioactive materials. Usually working with such materials is not very safe and there is always a risk of radioactive contamination. Thus regular leak test and special safety regulations are required. Therefore licensing and waste disposal are required which limits the acceptance of GCs equipped with such detectors in the market place. Another problem which is associated whit the use of a radioactive isotope is that in case of depositing unwanted materials, the interior of detection cell cannot be washed easily because it requires a special technology.
The other sources of electron formation that have been investigated include the following: (1) a thermo emitter such as in U.S. Pat. No. 6,023,169 ; (2) an activated photocathode such as in U.S. Pat. No. 7,015,467 ; (3) a hydrogen Lyman a emission such as paper by Wentworth, W. E. (J. Chromatograph. A, 112, P. 229,1975); (4) a rare gas resonance lamp source with an MgF2 window such as paper by Kapila, S. (J. Chromatograph. A, 259, P. 205,1983) and (5) a pulsed discharge in pure helium such as in paper by Huamin Cai (Anal. Chem. 1996,68,1233-1244). None of these non-radioactive sources has successfully replaced the radioactive material except the pulsed discharge detector. Operation of the latest one is in this way. An electrical discharge occurs in a flow of helium gas and generates high-energy photons coming from He2 emission in the range of 60-110 nm. The high-energy photons ionize a dopant gas to produce free electrons. These electrons move toward the collecting electrode along the biased path, forming a constant standing current. When electron attaching substances present in the effluent of GC the electron current is reduced.
Corona discharge is a relatively low-power electrical discharge that takes place at atmospheric pressure. It is generated by applying a high voltage to a sharp metal point that can create a stream of electrons, ionize the carrier gas molecules, and provide a high concentration of ions.
There are two types of corona discharges depending on the polarity of the electrode surrounded by corona which is negative or positive. The discharge current for negative corona in non-electron attaching gases such as nitrogen, helium and argon is exceptionally more than hundred times higher than that of the positive corona or even negative corona in for example air. The high current observed for the negative corona in pure nitrogen or helium can be explained considering the mechanism of the negative corona discharge. In a point-to-plane geometry, the electric field is very strong in the vicinity of the tip. At a sufficient voltage, electrons leaving the negative point are multiplied due to electron impact ionization of the gas molecules. Positive ions hitting the negative tip knock out more electrons and ensure the reproduction of electrons removed by the field.
In fact, when the needle is negative and the buffer gas is nitrogen, the needle produces a huge number of electrons such that the discharge current grows as high as 200 μA. The presence of any electron attaching substance suppresses the discharge and quenches the production of electrons. In this invention such type of corona discharge is used and the detector is specially designed such that the sample will not interfere with the discharge. Thus electrons are continuously produced regardless of the presence of electron attaching substances. Such electron source has already been used by Tabrizchi et all as an ionization source for negative ion mobility spectrometry (“A Novel use of Negative Ion Mobility Spectrometry for Measuring Electron Attachment Rates” M. Tabrizchi, A. Abedi, Journal of Physical Chemistry A, 108(30), (2004), 6319-6324).
In this invention the negative corona discharge in nitrogen or other non-electron attaching gases is used to provide a novel electron capture detector for GC. This electron source provides an ECD of quite robust, chemically inert and capable of operating up to 400° C.